ABSTRACTIsotopic and elemental proxies are useful for discerning the original compositions of ancient rocks subject to later diagenetic/thermal alteration, low-rank metamorphism, outcrop weathering, etc. Recent work in the Cariaco Basin (Lyons et al., 2003) has shown a high correlation between total organic carbon (TOC) content and Mo normalized to Al in these modern euxinic sediments:
Microlaminated, dark olive gray, silty clay (0 to 11.6 kyr BP)
TOC% = 1486*(Mo/Al) + 2.8; n = 13, r2 = 0.52
Mean rate of deposition = 36 cm/kyr
Distinctly microlaminated, dark olive gray, clayey mud
(11.6 to 14.5 kyr BP)
TOC % = 1622*(Mo/Al) + 0.22; n = 15, r2 = 0.89
Mean rate of deposition = 79 cm/kyrHere, we use these relationships to estimate the original TOC contents of ancient black shales with overall characteristics similar to those of the modern Cariaco sediments. These "Group IV" black shales as defined by Quinby-Hunt and Wilde (1996), are characterized by relatively high concentrations of V, Mo, and Co but low Mn contents. The Cariaco regressions and those from the Carboniferous of Iowa and the Devonian of New York were used to estimate the 'original' TOC contents for Lower Ordovician black shales of the Baltica and Avalonia plates, where Corg values were not taken. For individual samples, the Carboniferous regression produced TOC values approximately double that derived from the regression equation of the Cariaco Basin lower anoxic zone. Such variation among the results from the four regressions suggest that there is no universal proxy for TOC using Mo/Al .
These calculated TOC values, however, are consistent with the higher levels of primary production predicted from the paleogeographic settings of these intervals. In general, the Mo proxy for original TOC content, while approximate, works for oxygen-deficient sites of deposition where other proxies for carbon loss, such as organic-carbon/pyrite-sulfur ratios in normal (oxic) marine shales, do not apply. Estimates of original TOC from Mo content in samples spanning the geologic record, combined with paleogeography and paleoecology, should be useful in estimating pathways of carbon synthesis and remineralization in ancient oceans and initial hydrocarbon potential of petroleum source rocks.
KEYWORDS: Black Shales, Chemical Proxies, Organic Carbon, Molybdenum, Baltica, Avalonia
1. INTRODUCTION
With the growing number of whole-rock geochemical data now available, elemental proxies are becoming increasingly valuable in the interpretation of early depositional histories of ancient rocks. With time, the original composition of rocks will change due to weathering; pore-fluid exchange and associated rock-water interactions; secondary mineral overprints; thermal processes, including metamorphism; and in general with the varying chemical conditions seen by sediments as they lithify and become deeply buried. Even black shales, which because of their low porosities and permeabilities after early diagenesis should be impervious to compositional alteration over the short term, are relatively open systems over geologic timeframes.
Berner and Raiswell (1983) noted the unsuitability of weathered rocks for carbon and sulfur analyses, conclusion corroborated by the recent work of Petsch et al. (2000, 2001). Zaback et al. (1993) and many others have shown that processes such as sulfate reduction can occur in sediments long after burial, thus modifying the primary (water-column) geochemical signals and even the records of early diagenesis (e.g., Lyons et al., 2003). Canfield (1994) discussed the general problem of carbon preservation in marine rocks and the degree to which it can vary as a function of depositional redox and sedimentation rate. Most recently, Kennedy et al. (2001), based on a study of two black shale locations of Cretaceous age, suggested that organic carbon (Corg) absorption onto clay mineral surfaces is a major controlling factor in its burial and preservation. Peucker-Ehrenbrink and Hannigan (2000) and Jaffe et al. (2002) investigated mobility of platinum group elements and Corg during weathering of black shales and found a large (77%) decrease in TOC near the surface (Jaffe et al., 2002), supporting the contentions of Berner and Raiswell (1983), Petsch et al. (2000, 2001) and others. The thermal loss of Corg is well known (e.g., Raiswell and Berner, 1987). Therefore, despite our robust, integrated models for black shale deposition (e.g., Stow et al., 2001; Sageman and Lyons, in press), paleoenvironmental reconstructions are often limited by our ability to discern primary geochemical compositions.
2. THE MOLYBDENUM PROXY
Lyons et al. (2003) reported on results from Ocean Drilling Program Site 1002 in the Cariaco Basin, a modern anoxic-sulfidic (euxinic) pull-apart basin off the coast of Venezuela (Dean et al, 1999; Muller-Karger et al., 2000; Peterson et al., 2000; Yarincik et al., 2000; Scranton et al., 2001). Lyons et al. (2003) reported surface sediments dating back to approximately 15 kyr before present. Earlier, Wilde et al. (2001) recognized correlations between Corg and molybdenum in these data (Fig. 1) and used the resultant regression equations to estimate the original minimum Corg contents of a suite of Lower Paleozoic black shales. Such a correlation is not unexpected in anoxic sediments. For example, Brumsack (1986) reported a high correlation between Corg and V, Mo, and Zn for Cretaceous black shales from Cape Verde Basin drill cores, and Werne et al. (2002) showed striking covariance between Corg and Mo/Al ratios in Devonian euxinic shales from western New York. Nijenhuis et al., 1999) discussed the general Corg-trace metal relationship using Pliocene examples. The present paper is an expansion on our earlier abstract and a further exploration of the relationship between Corg and Mo in fine-grained sediments deposited beneath oxygen-deficient waters.
The positive correlation observed between concentrations of Corg and Mo in many black shales and sediments of modern, oxygen-deficient marine basins is well known. In some instances this covariation may be partially or entirely an artifact of dilution by high but variable fluxes of biogenic sediment (opal or calcium carbonate). For example, Corg and Mo contents in Unit 1 in the modern Black Sea rise and fall sympathetically with varying CaCO3 dilution (Lyons, preliminary unpublished results), but the correlation breaks down when Mo is normalized to Al content to remove spurious enrichment trends. By contrast, Mo/Al ratios and Mo mass accumulation rates (mass Mo area-1 time-1) at many other sites suggest a true mechanistic linkage between the delivery/burial of Corg and Mo. This coupled accumulation of Corg and Mo, where observed, is generally coherent over the Phanerozoic, but it is clear that no universal linear relationship exists. In ancient sediments, shifts in the slope and scatter in the data can reflect selective loss of Corg or remobilization of Mo during burial and weathering. However, the results of the present study reveal that primary differences can occur over comparatively short time intervals even within a single anoxic basin.
Ultimately, we agree with past workers that the availability of dissolved sulfide is a critical control in Mo sequestration (Helz et al., 1996), and parallel accumulation of Corg may simply drive the system capacity to generate hydrogen sulfide on both local and basin scales. A number of workers have suggested, however, that the requisite high levels of sulfide can occur either in pore waters or in the water column (reviewed in Lyons, 2003, and Sageman and Lyons, in press) thus limiting the utility of Mo as an unambiguous proxy for euxinicity. Also, recognizing that persistent availability of dissolved sulfide is a function of bacterial production as well as H2S loss through pyrite formation, supplies of reactive Fe become an essential control in Mo enrichment (e.g., Meyers et al., submitted).
In addition to sulfide production, there is likely a more direct coupling between Corg and Mo burial through reactions between Mo-bearing dissolved species (e.g., thiomolybdate) and organic matter (Helz et al., 1996). If so, the types and relative amounts of marine and terrestrial organic matter can be critical, giving rise to intrabasinal variation in Mo (vs. Corg) distributions in, for example, Carboniferous shales (Coveney et al., 1991; Helz et al., 1996; Cruse and Lyons, this volume; see review in Lyons et al., 2003). The Cariaco data discussed here (Figs 1 and 2 and in Lyons et al., 2003) argue against a direct coupling between Mo and pyrite accumulation (compare Huerta-Diaz and Morse, 1992). Finally, Kao et al. (2001) reported on the mineralogy of a mixed-layer carbon-molybdenum sulfide phase from Cambrian metalliferous black shales in China. The broad relevance of this relationship is unknown, but such phases may play a role in hosting the carbon-molybdenum pair.
Despite these and other complications and the potential for scatter in patterns of Mo versus Corg burial, ratios of Corg to Mo/Al show general similarities among many temporally and spatially diverse organic-rich sediments (e.g., Fig. 3). Overall, the correlation should be strongest where sulfide concentrations and/or organic matter type are most favorable to Mo accumulation. Interestingly, however, the data in Figure 3 for Paleozoic shales, which have not experienced appreciable heating during burial, show Corg enrichment rather than loss relative to the Cariaco modern baseline. This unexpected result likely reflects intersite differences among original Corg-Mo relationships in sediments that have not experienced appreciable Corg loss since deposition.
Clearly, there is no universal relationship between Corg and Mo in oxygen-deficient settings, but there is often at least general agreement suitable for our purpose here (Fig. 3), which is to approximate Corg loss in highly altered shales. Nevertheless, some modern and ancient euxinic settings fail to show covariance between Mo/Al and Corg. These differences are beyond the scope of this paper but likely reflect a complex combination of multiple primary and secondary controls, such as (1) the magnitude of sulfide availability, including water-column H2S concentrations that may have been less intense and less persistent than traditionally interpretetions for black shale paleoenvironments (Murphy et al., 2000; Sageman and Lyons, in press); (2) the relative roles of pore-water versus water-column sulfide availability; (3) the amount and type of organic matter present, including relative terrestrial versus marine inputs; (4) Mo remobilization during weathering and higher-temperature burial alteration; (5) local and global differences in seawater Mo chemistry; (6) Corg contributions via in situ production of microbial biomass; and (7) secondary Mo overprints related to hydrothermal processes.
3. THE CARBON-SULFUR ANALOG
In an effort to quantify the loss of organic carbon during thermal maturation, Raiswell and Berner (1987) exploited the commonly observed covariation between Corg and pyrite sulfur (Spy) observed in many modern and ancient normal (oxic) marine sediments. Because pyrite formation beneath oxic bottom waters in restricted to the sediments (i.e., is purely diagenetic), and because H2S production is thus controlled by the local availability of metabolizable Corg, Spy concentrations are frequently limited by the concentration of Corg if adequate reactive Fe is present (Berner, 1984; compare Raiswell and Canfield, 1998). Such a coupling between Corg and Spy is expressed as the well-known covariation between the abundances of the two components in normal marine sediments, with a zero sulfur intercept (modern C/S ratio of ~2.8; Berner, 1984; Morse and Berner, 1995). Raiswell and Berner (1986) explored the principal controls on this ratio over time and later concluded (Raiswell and Berner, 1987) that dramatic deviance from the predicted temporal trends could be attributed to selective Corg loss during thermal maturation of the organic matter. In short, this was a tool for assessing Corg loss much like that proposed here.
One complicating consideration for the C-S method is that the positive linear relationship between Corg and Spy exploited by Raiswell and Berner (1987) breaks down under the oxygen-deficient conditions that prevail in the Cariaco Basin, the Black Sea (Lyons and Berner, 1992; Lyons, 1997), and during deposition of many black shales (Raiswell and Berner, 1985). Under such conditions, iron limitation, rather than Corg supplies, becomes the dominant control on pyrite formation. We suggest, therefore, that the C/S and Corg-Mo/Al approaches are complementary rather than overlapping, and each has a unique paleoenvironmental relevance. It is also important to note that, like the Corg-Mo relationship, the strength of the C-S method is limited by the appreciable scatter surrounding the modern mean C/S ratio of 2.8 and the mean ratios spanning geologic time. Many of the caveats outlined here for the Corg-Mo relationship, such as the importance of terrestrial versus marine organic inputs, also apply to the C-S system and have been addressed in detail in past studies of sedimentary pyrite formation (Raiswell and Berner, 1986). However, the fruits of two decades of research centered on C-S relationships far exceed our nascent understanding of coupled Corg-Mo burial.
4. RESULTS
Figure 1 is a composite profile of the material sampled in the Cariaco Basin (see Lyons et al., 2003, for details). As shown, the correlations between Corg and Mo/Al ratios divide into two, sedimentologically and geochemically distinct intervals in the core: (1) microlaminated dark olive gray silty clay spanning the upper ~ 420 cm (0 to 11.6 kyr BP), with a mean rate of sedimentation of ~36 cm/kyr, underlain by (2) distinctly microlaminated dark olive gray clayey mud about 230 cm thick (11.6 to 14.5 kyr BP), with a mean sedimentation rate of ~79 cm/kyr. Below these zones, the sediment are bioturbated, indicating oxic overlying bottom waters, and the correlation does not hold (see Lyons et al, 2003, and references therein for sedimentological, geochemical, and paleoceanographic details). The molybdenum values on the graph are normalized to aluminum as a proxy for detrital clay content. This way, the time-varying effects of dilution by calcium carbonate and biogenic opal are removed. Van der Weijden (2002) discussed the potential problems associated with data normalization, but because we are simply looking for Mo enrichment beyond the detrital flux, and our Mo/Al trends are generally consistent with the Mo accumulation rates calculated by Dean et al. (1999), we feel justified in using this procedure.
Figure 2 shows the linear regressions with the accompanying statistics for the two anoxic zones identified in the post-glacial Cariaco Basin. It should noted that the lower anoxic zone with the higher rate of sedimentation has a higher r2 value than the upper zone (0.9 vs. 0.5). One possibility for the two different trends in the Cariaco sediments is the relationship between the deeper zone and the high primary productivity inferred over this interval (Peterson et al., 1991; Werne et al, 2000; Lyons et al., 2003). Among other things, such an interval might have had higher associated hydrogen sulfide concentrations in the water column and in the sediments, facilitating the more-effective scavenging of Mo. The sulfur and iron expressions of such differences in H2S conditions would be muted by the persistent record of Fe-limited, water-column pyrite formation throughout the section (Lyons et al., 2003; Werne et al., 2003). The discordant trends may also stem from differences in rates of sedimentation and other variation in depositional history, including relative amounts of turbiditic sediment, temporal variation in the types of dominant organic material produced and deposited in the basin (Werne et al., 2000), and overall persistence of a given set of environmental conditions in the water column.
5. APPLICATION TO ANCIENT SEDIMENTS
Based on the black shale database of Quinby-Hunt et al.(1989), Quinby-Hunt and Wilde (1991, 1994, 1996) postulated four geochemical groups for black shales varying as a function of pH and redox conditions and identified by characteristic elemental abundances. Only two of the four groups had significant Mo content. These groups are characterized by relatively low Mn and Fe contents, relatively high V, Mo, and Co, and were considered the more anoxic of the black shale facies. Based on their overall geochemical characteristics, we group the microlaminated Cariaco Basin samples with as borderline Group IV on a carbonate free basis. The two separate regressions identified for the two facies seen in Figure 1 demonstrate that Mo in black shales is expressed by more than one simple relationship with TOC content, which is not surprising given the multiple controlling factors on Mo accumulation in marine systems (Morford and Emerson, 1999, Meyers et al, submitted).
Table 1 shows the predicted values of Corg for samples in a black shale database from the Lower Ordovician of the Baltica and Avalonia terranes of the Iapaetus Ocean, calculated from the regressions shown in Fig. 3. Paleogeographic reconstructions (Fig. 4) of these terranes place Baltica samples at approximately 30 degrees off a western coast and the Avalonia samples at 60 degrees south importantly both in regions of predicted high primary productivity (Wilde et al., 1989, 1990). The Laurentia samples from Quebec (Levis) are on the same latitude as the Baltica samples but are not from a coastal region of paleo-upwelling. Samples from the same continental masses but off the productivity highs do not show Group IV characteristics and are therefore excluded from the calculations.
The highest calculated TOC values are from Swedish core material, thus re-enforcing the Berner and Raiswell (1983) dictum on the difference in preservation between core and potentially weathered outcrop samples. In this case, because we are calculating TOC from Mo content, the calculations appear to be compromised by the weathering effects on Mo. Thus, while our method permits estimates of original TOC contents, it may be best applied to Corg loss through burial processes less extreme to metal remobilization than surficial weathering. Which regression is most applicable to these Ordovician rocks is not clear. However, the use of all in Table 1 allows us to bracket the range of possible original Corg contents. The wide range of Corg (reported as TOC %) values certainly indicate that we have not achieved a method for universal application to any black shale without some additional regression criteria.
6. SUMMARY
Proxies for original Corg content in black shales based on an extrapolation of Mo/Al trends seen in modern material from the Cariaco Basin and from two Paleozoic locales has been applied to black shales from the Lower Ordovician. The significant variation in Corg calculated values using different regressions indicates that more work is needed in classifying black shales to match any proxy regression with a particular shale. However, this technique, once refined, has the potential to "rescue" many older data sets for which TOC data were not generated, but where metals such as Mo were analyzed, and for which the C-S normal marine proxy does not apply. Such approaches are useful in evaluating petroleum potential of reservoir shales by comparing present TOC values with the proxy estimates, thus constraining thermal Corg loss. Gelinas et al. (2001) reaffirmed the view that anerobic sediments, such as examined here, "were essential for the generation of petroleum source rocks". This technique for assessing potential hydrocarbon yield may be used best for core material, which will not have experienced weathering and thus recent surficial oxidation of Corg. Also, metal remobilization associated with surficial processes will not be a factor (see Morford and Emerson, 1999). Finally, proxy values for TOC when viewed in an integrated geochemical, paleoceanographic, and paleogeographic context could aid in reconstructions of areas of high productivity as well as other paleoecologic processes. For example, Ganeshram et al. (1999) found organic carbon burial in the modern environment off NW Mexico was governed mainly by productivity rather than overlying oxygen content in the water column, as has been suggested for many other settings, including the Holocene Black Sea (Calvert and Karlin, 1998). In future papers, we will investigate other proxies such as vanadium, which are also characteristically enriched in Type IV black shales. We also recognize that this technique can result, at best, in a minimal approximation of the TOC values without a more comprehensive mechanistic understanding of the relationship between Corg accumulation and Mo enrichment, including the primary controls on any temporal and spatial variability in the Corg-Mo correlation. On the short term, we would do well to explore this correlation in a variety of other settings, modern and ancient, with the hope of finding consistencies of broad temporal and spatial relevance.
ACKNOWLEDGEMENTS
The authors thank Prof. Dr Hans Brumsack for sharing his data and two external reviewers for their time and useful comments. TWL acknowledges support from NSF grants EAR-9875961 and EAR-9725326 and from JOI-USSSP. This is Contribution 03-01 of the Pangloss Foundation.
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